1. Field of the Invention
The present invention concerns a thermionic emission device (in particular for use in an x-ray tube) with an indirectly heated primary emitter that is fashioned as a flat emitter with an unstructured primary emission surface, and with a heating emitter that is fashioned as a flat emitter with a structured heat emission surface, wherein the primary emitter and the heating emitter respectively have at least two terminal lugs, and wherein the primary emission surface and the heat emission surface are aligned essentially parallel to one another.
2. Description of the Prior Art
A thermionic emission device of the above type, that is used as a cathode in an x-ray tube, is known from WO 2008/047269 A2. In this emission device an indirectly heated, unstructured, flat emission surface, having at least two fixing elements that lie in the plane of the emission surface and through which an electrical current can be conducted, is structurally fixed in a unit surrounding it. This emission surface is heated by electron bombardment from a directly heated flat emitter with a structured emission surface through which a heating current is directed.
An unstructured emission surface means a flat, essentially homogenous emission surface without slits or similar interruptions. An emission surface that is interrupted by slits or has a serpentine conductor trace is designated as structured.
The size of the focal spot at which the electrons accelerated from the cathode in the direction of the anode strike the anode is of prominent importance for the quality of the x-ray radiation generated by an x-ray tube. The size of the focal spot can be disadvantageously affected by the design of the electron-emitted components.
For example, if a directly heated flat emitter for electron emission is used to generate x-ray radiation, its emission surface is generally structured and has slits or similar interruptions. A serpentine structure of the conductor trace is generally necessary so that the heating current flows through the entire emission surface and heats uniformly. The electrical field lines then extend into the interstices in the emission surface that are produced by the slits and thereby have a component tangential to the emission surface. Since the electrons essentially follow the field lines on their path to the anode, the optical aberration of the electron source is intensified and the focal spot is enlarged in an unwanted manner. For this reason the aforementioned design with an indirectly heated, unstructured emitter is generally preferred.
The emission device known from WO 2008/047269 A2 exhibits the disadvantage that a thermal expansion of the terminal lugs (also designated as emitter legs) can lead to a deflection of the primary emission surface, and therefore to an unwanted defocusing of the electron beam.